1. Field of the Invention
The present invention relates to alkali rechargeable batteries in which an anode principally comprising a magnesium-nickel alloy is used and a process for producing said rechargeable batteries.
2. Related Background Art
In recent years, the global warming of the earth because of the so-called greenhouse effect due to an increase in the content of CO2 gas in the air has been predicted. For instance, in thermal electric power plants, thermal energy obtained by burning a fossil fuel is being converted into electric energy, and along with burning of such fossil fuel, a large amount of CO2 gas is being exhausted in the air. Accordingly, in order to suppress this situation, there is a tendency of prohibiting to newly establish a thermal electric power plant. Under these circumstances, so-called load leveling practice has been proposed in order to effectively utilize electric powers generated by power generators in thermal electric power plants or the like, wherein a surplus power unused in the night is stored in rechargeable batteries installed at general houses and the power thus stored is used in the daytime when the demand for power is increased, whereby the power consumption is leveled.
Now, for electric vehicles which do not exhaust any air polluting substances such as CO2, NOx, hydrocarbons and the like, there is an increased demand for developing a high performance rechargeable battery with a high energy density which can be effectively used therein. Besides, there is also an increased demand for developing a miniature, lightweight, high performance rechargeable battery usable as a power source for portable instruments such as small personal computers, word processors, video cameras, and cellular phones.
Under such circumstances, there are known so-called nickel-metalhydride rechargeable batteries belonging to an alkali rechargeable battery, which have an anode comprising a hydrogen absorption alloy having a performance of absorbing and storing hydrogen (this performance will be hereinafter referred to as xe2x80x9chydrogen-absorbing and storing performancexe2x80x9d) and which has a high capacity.
As the hydrogen absorption alloy which constitutes the anode of such nickel-metalhydride rechargeable battery, there have been proposed mish metal series alloys represented by Mm(Nixe2x80x94Coxe2x80x94Mnxe2x80x94Al)5, transition metal series alloys represented by Tixe2x80x94Zrxe2x80x94Nixe2x80x94Vxe2x80x94Crxe2x80x94Coxe2x80x94Mn, and magnesium-nickel alloys, i.e., Mg2Ni alloy and MgNi alloy. Of these, such mish metal series alloy and such transition metal series alloy have been used as an electrode material to constitute the anode of the nickel-hydride rechargeable battery.
Now, in The 37th Battery Symposium in Japan, p.p. 389-390 (1996) (hereinafter referred to as xe2x80x9cdocument 1xe2x80x9d), there is described about electrode characteristics of a Mgxe2x80x94Mg2Ni alloy powder prepared with a mechanical grinding method. Particularly in document 1, there is described that an anode was prepared using a composite powder obtained by subjecting a mixture of Mg2Ni and Ni to a mechanical treatment using a planetary ball mill, a nickel-metalhydride battery was prepared using said anode, and said nickel-metalhydride battery was subjected to charge-and-discharge cycle test under condition with a temperature of 25xc2x0 C. to observe electrochemical characteristics of the battery. And document 1 describes that at the first charge-and-discharge cycle, there was obtained a large discharge capacity of 750 mAh/g. However, as apparent from FIG. 2 on page 390 of document 1, it is understood that the discharge capacity is significantly decreased as the number of the charge-and-discharge cycle is increased. Hence, it is understood that there was not obtained a magnesium-nickel alloy electrode which stably and continuously provides a high discharge capacity when used as the anode of a rechargeable battery.
Besides, Japanese Unexamined Patent Publication No. 275631/1998 (corresponding to U.S. Pat. No. 6,040,087) [this publication will be hereinafter referred to as xe2x80x9cdocument 2xe2x80x9d] discloses an electrode member for an anode of an alkali rechargeable battery, comprising a core layer (a first layer) of a hydrogen absorption alloy comprising, for instance, nickel and magnesium, a metal oxide layer (a second layer) provided on the surface of said core layer and having a function to prevent the oxidation of the hydrogen absorption alloy constituting the core layer, and a metal dispersion layer (a third layer) provided on the surface of said metal oxide layer, comprising a transition metal element provided having a property to activate hydrogen. Document 2 describes that according to said electrode member, it is possible that hydrogen is absorbed by the metal dispersion layer situated on the outermost surface side to efficiently generate atomic hydrogen by virtue of catalytic action of the transition metal element, and said atomic hydrogen and hydrogen ion passed through the metal oxide layer are stored in the hydrogen absorption alloy constituting the core layer. Document 2 further describes that a nickel-metalhydride rechargeable battery having an anode prepared using aforesaid electrode member has advantages such that the quantity of the hydrogen stored in the anode upon charging is increased, the charging efficiency, the charge capacity and the discharge capacity are improved, and the battery has a prolonged cycle life (a prolonged charging and discharging cycle life). Although the electrode member disclosed in document 2 provides such various advantages as above described, there is still a demand for improving said electrode member to have more improved characteristics or for providing other adequate electrode member having excellent characteristics even when it has a constitution which is different from that of the electrode member disclosed in document 2.
The present invention has been accomplished in view of the foregoing situation in the prior art for alkali rechargeable batteries represented by nickel-metalhydride rechargeable batteries.
An object of the present invention is to provide a high performance alkali rechargeable battery having an improved charge-and-discharge capacity and a prolonged cycle life (a prolonged charging and discharging cycle life).
Another object of the present invention is to provide an alkali rechargeable battery comprising at least an anode, a cathode, and an electrolyte or an electrolyte solution, said anode principally comprising a magnesium-nickel alloy capable of absorbing and storing hydrogen therein and releasing the hydrogen stored therein in the electrochemical reaction, wherein said magnesium-nickel alloy constituting said anode has a surface having a coat layer provided thereon, said coat layer comprising an insulating material which is not dissolved in an aqueous solution of a hydroxide of an alkali metal as said electrolyte solution, which restrains a reaction which forms a magnesium hydroxide when said magnesium-nickel alloy contacts with said aqueous alkali solution, and which allows contacts with said aqueous alkali solution, and which allows hydrogen or hydrogen ion to pass therethrough. Said insulating material comprises a hydrous oxide, a hydroxide or an ionomer-containing polymer. In this rechargeable battery, said magnesium-nickel allow constituting said anode is prevented from directly contacting with and reacting with said aqueous alkali solution as the electrolyte solution, whereby the high hydrogen-absorbing and storing performance of the magnesium-nickel alloy which contains amorphous phases by nature is prevented from being deteriorated. Because of this, the rechargeable battery has an improved charge-and-discharge capacity and a prolonged cycle life.
A further object of the present invention is to provide a process for producing aforesaid rechargeable battery, characterized by including at least a step of forming the anode of the rechargeable battery by coating a magnesium-nickel alloy constituting said anode using at least an aluminum material, or by subjecting (i) an electrode formed using a magnesium-nickel alloy powder or (ii) a magnesium-nickel alloy whose surface is coated with an aluminum material to an anodization treatment by way of applying a prescribed voltage between any of said electrode (i) or said magnesium-nickel alloy (ii) as a positive electrode and a counter electrode in an electrolyte solution, or by coating (a) a magnesium-nickel alloy powder or (b) an electrode formed using a magnesium-nickel alloy powder by an ionomer-containing polymer, wherein if necessary, the surface of said magnesium-nickel alloy powder (a) having said ionomer-containing polymer coated thereon is further coated by another ionomer-containing polymer.
The process makes it possible to relatively easily form an anode using an inexpensive starting material. This situation enables one to produce a high performance alkali rechargeable battery at a reasonable production cost.